A solar cell manufactured by the prior art technology is described with reference to its cross-sectional view (FIG. 1), front surface configuration (FIG. 2), and back surface configuration (FIG. 3). In general, the solar cell includes a p-type semiconductor substrate 100 of silicon or the like in which an n-type dopant is diffused to form an n-type diffusion layer 101 to define a p-n junction. On the n-type diffusion layer 101, an antireflection film 102 such as SiNx film is formed. On the back surface of p-type semiconductor substrate 100, aluminum paste is coated over substantially the entire surface and fired to form a back surface field (BSF) layer 103 and an aluminum electrode 104. Also on the back surface, a broad electrode 106, known as bus bar electrode, is formed for current collection by coating a conductive paste containing silver or the like and firing. On the light-receiving surface side, finger electrodes 107 for current collection and broad electrodes 105, known as bus bar electrodes, for collecting current from the finger electrodes are arranged in a comb-shaped pattern so as to cross at substantially right angles.
In the manufacture of solar cells of this type, electrodes may be formed by various methods including evaporation, plating, printing and the like. The front finger electrodes 107 are generally formed by the printing/firing method to be described below because of ease of formation and low cost. Specifically, a conductive paste obtained by mixing silver powder, glass frit, organic vehicle, and organic solvent as main ingredients is generally used as the front electrode material. The conductive paste is coated by screen printing process or the like, and fired at high temperature in a firing furnace to form the front electrode.
The screen printing process is described below.
The screen printing process uses a screen printing plate which is prepared by providing a mesh fabric 110 of orthogonally woven warp and weft filaments, coating the fabric with a photosensitive emulsion 111, exposure, and removing parts of the emulsion to define a substantially rectangular pattern hole (FIG. 4). The screen printing plate is placed over the work to be printed. A print paste (ink) is rested on the screen printing plate and spread over the pattern. A flexible blade known as printing squeegee 112 is traversed at a suitable squeegee hardness (60 to 80 degrees), squeegee angle (60 to 80 degrees), pressure or applied pressure (0.2 to 0.5 MPa), and printing speed (20 to 100 mm/sec) for thereby transferring the print paste to the work to be printed through the pattern hole. The print paste applied to the work to be printed is then dried to form a printed pattern.
Immediately after the print paste falls down through meshes in the pattern hole where filaments are absent and attaches to the work to be printed, the print paste remains unattached to portions corresponding to warp and weft filaments in the pattern hole. Later, the print paste attached to the portions corresponding to meshes starts flowing, resulting in a continuous printed pattern of uniform thickness.
As described above, the screen printing process is such that the print paste filled in the pattern hole on the screen printing plate is transferred to the work to be printed by traversing movement of the printing squeegee (or blade) whereby the same pattern as the pattern hole defined on the screen printing plate is formed on the work to be printed.
The contact resistance between the front finger electrode 107 formed by the above process and the silicon substrate 100 and the interconnect resistance of the electrode largely affect the conversion efficiency of a solar cell. To gain a high efficiency (low cell series resistance, high fill factor (FF)), the contact resistance and the interconnect resistance of the front finger electrode 107 must have fully low values.
Also, the electrode area must be small so that the light-receiving surface may take in as much light as possible. To improve short-circuit current (Jsc) while maintaining the FF, the finger electrode must be formed such that it may have a reduced width (thin) and an increased cross-sectional area, that is, a high aspect ratio.
While various methods are used to form solar cell electrodes, known methods of forming ultra-fine lines having a high aspect ratio include a method of forming grooves in a cell and filling the grooves with paste (JP-A 2006-54374) and a printing method based on inkjet printing. However, the former method is undesirable because the step of forming grooves in a substrate can cause damage to the substrate. Since the latter, inkjet printing method is designed to apply pressure to liquid to inject droplets through a thin nozzle, it is suitable to form fine lines, but difficult to gain a height.
On the other hand, the screen printing method is a low-cost, high-productivity method because formation of a printed pattern is easy, damaging of the substrate is minimized by adjusting the applied pressure, and the working rate per cell is high. If a conductive paste having high thixotropy is used, an electrode retaining the shape as transferred and having a high aspect ratio can be formed.
As discussed above, the screen printing method is more suitable to form high-aspect-ratio electrodes at low cost, than other printing methods.
However, when fine lines are printed using the above method, there arise a problem that the connection between bus bar electrode and finger electrode becomes very thin and at the worst, broken. If the finger electrode on the light-receiving side is locally thinned or even broken, that portion becomes a controlling factor of resistance, resulting in a drop of fill factor.
The cause of breakage is a difference in film thickness at the connection between bus bar electrode and finger electrode. In screen printing, the buildup of paste is in proportion to the size of an opening. Namely, a large buildup of paste is given for the bus bar electrode corresponding to a large opening whereas a small buildup of paste is given for the finger electrode corresponding to a small opening. Thus a difference arises in film thickness between the bus bar electrode and the finger electrode. If the electrodes are fired in this state, breakage occurs at the boundary between bus bar electrode and finger electrode because the bus bar electrode with a larger buildup undergoes a more shrinkage. If the difference is small, there arises a phenomenon that the connection between bus bar electrode and finger electrode becomes very thin.
Further, in the screen printing process, the printing direction (traversing direction of a printing squeegee) also becomes a factor of promoting breakage. For preventing breakage of a finger electrode, the screen printing plate 1 is generally patterned such that the printing direction and the finger electrode opening 2 are substantially parallel, and the printing direction and the bus bar electrode opening 3 are substantially perpendicular (FIG. 8). With this design, the electrodes as printed are configured such that the width of the connection between bus bar electrode 13 and finger electrode 12 positioned on the upstream printing side with respect to finger electrode 12 is very narrow (FIG. 9). Such narrowing is outstanding particularly when fine lines are printed. This is because at the connection between finger electrode opening 2 and bus bar electrode opening 3, the printing squeegee falls in bus bar electrode opening 3, resulting in a less buildup of paste at this connection. In contrast, the width of the connection between bus bar electrode 13 and finger electrode 12 positioned on the downstream printing side tends to be broad because of a more buildup of paste (FIG. 9). Notably the plate includes blocked or masked zones 5.
In addition, a saddle phenomenon is likely to occur since the bus bar electrode opening 3 is considerably wider than the finger electrode opening 2 and the squeegee 112 traverses across the plate perpendicular to the bus bar electrode opening 3 as mentioned above. The saddle phenomenon is that when a wide open portion like bus bar is printed, the open portion is pressed by the squeegee 112 (FIG. 5), and a central portion 113 is dented deeper than edges of paste in the width direction of bus bar electrode (FIG. 6). Occurrence of a saddle phenomenon brings a difference between the height of bus bar electrode at its edge in its width direction and the height of finger electrode. Since the bus bar electrode edge with a more buildup has a higher shrinkage factor during electrode firing, the connection between bus bar electrode 13 and finger electrode 12 can be broken 114 (FIG. 10). It is noted that in FIG. 10, the broken line denotes the connection between bus bar electrode 13 and finger electrode 12.
Even when a finger electrode and a bus bar electrode are separately printed, a saddle phenomenon occurs at the bus bar electrode, failing to prevent breakage at the connection between bus bar electrode and finger electrode.
To solve the above problem, JP-A 2009-272405 discloses broadening of the connection between bus bar electrode and finger electrode. On use of this method, however, blurs or clumps form because the connection between bus bar electrode and finger electrode is extremely thick. This gives rise to problems like an increased shadow loss and deteriorated properties. Since the solar cell is, as a matter of course, a device which is used under sunlight, there are many chances to public view, unlike other semiconductor devices. Accordingly, not only the performance, but also the appearance is very important for the solar cell. The method of the above patent has the problem that since the connection between bus bar electrode and finger electrode is thick, the finger electrode becomes discontinuous in width, detracting from the aesthetic appearance.
It is also known to prevent the squeegee from falling in the bus bar opening by performing screen printing with the installation position of the screen printing plate rotated to an angle other than multiples of 90° relative to the squeegee travel direction (FIG. 7). This method, however, has the problem that since the squeegee travel direction is not parallel to the finger opening, the finger electrode is blurred, failing in precise printing.